The present invention relates generally to the field of metal-elastomer composites and to the problem of reinforcement of polymers, elastomers and especially rubbers with metal elements such as elongated steel elements. More particularly the present invention relates to a coated metal reinforcement element for polymeric or elastomeric materials, as well as methods for the coating of metal surfaces with organic materials to improve the polymer-to-metal bond.
Composites of metal and elastomers wherein reinforcing elements of steel or steel alloys are bonded to rubber are widely applied in a variety of products, including conveyor belts, heavy duty hoses, pulley belts and pneumatic tires.
In today""s tire technology, metal reinforcements are generally coated with a brass layer that undergoes a chemical reaction during curing, with the sulfur curable rubber composition leading to a surface layer predominantly consisting of a copper sulfide layer. This copper sulfide layer facilitates adhesion by mechanical and/or chemical interaction with the cured rubber. Rubber compositions of adhesion rubbers in today""s technology have to be fine tuned to support the very specific chemical requirements of adhesion layer formation, and the curing system has to be adjusted to the delicate kinetics of copper sulfide formation in order to reach the optimum balance between size and structure of the copper sulfide dendrons. Rubber compositions that are fine tuned for a variety of uses, such as skim compounds in tires, are high cost materials due to the large amount of insoluble sulfur and expensive adhesion promoters like cobalt containing chemicals contained therein. Furthermore, compounding for adhesion rubbers is currently a compromise between the demands of the adhesion system and other demands like the resistance against aging phenomena, corrosion, fatigue and durability under the high shear forces acting in tires in service.
On the other hand there have also been proposed numerous adhesion systems where organic adhesives are interposed between the metal and the elastomer or plastic material to effect an improved metal-to-elastomer/plastic bond. The WO 97/17144 A1 patent, for example, teaches a composition suitable for treating metal surfaces prior to bonding of the metal surfaces to other materials including metals, rubber, glass, polymers, sealants, and coatings, to enhance bond strength and to prolong useful life in corrosive environments. The composition comprising an organoalkoxysilane is capable of crosslinking when applied to the metal surface to form an adherent coating. It is also capable of bonding with the material to be bonded to the metal surface via functional groups, thereby forming a strong adherent bond.
Especially for materials under high stress in service, the use of a thin organic adhesive layer alone may, however, not suffice because there is still the problem of adhesion loss caused by high dynamic loads and shear forces acting across the interface between the rigid surface of the metal with the adhesive and the polymer matrix.
Patent abstracts of Japan vol. 8, no 37, JP 58193134A disclose the coating of a steel cord with liquid rubber bearing COOH or OH functional groups and a vulcanization of said coated cord embedded in a rubber. For this purpose the steel cord is at first transferred into a liquid rubber bath containing preferably a liquid polyisoprene rubber with an average viscosity molecular weight of 15000-50000. Although the liquid rubber is of comparatively low molecular weight, the viscosity of the liquid rubber bath is too high to give thin coating layers. This leads to weak boundary layers with the embedding rubber.
FR 2320974 A discloses a coating for a steel reinforcement element of a first layer of an organosilane and a second layer of an organic RPL adhesive i.e. an aqueous emulsion of a resorcinol-formaldehyde resin and a rubber-containing latex, which is cured thereafter. This product can be used as a reinforcement element for rubber products. The curing of the RFL adhesive makes it impossible to get a smooth boundary layer between the coating and the rubber. Moreover the non-reactive rubber-latex is only held mechanically within the structure of the cross-linked resin and cannot contribute to the bonding between the coating layer and the rubber to be reinforced.
FR 2271036 A discloses a coating for a steel reinforcement element of a first layer of a vinyl polymer comprising OHxe2x80x94 and/or COOH-groups and a second layer of a composition of an organic RFL adhesive and a styrene-butadiene vinyl pyridine terpolymer, where both layers are finally heat treated. Therefore, the same drawbacks as described for FR 2320974 A arise.
It is therefore an object of the invention to find an adhesion promoting system between a metal surface and an elastomer that is able to result in superior resistance against aging, corrosion, dynamic loads and shear forces acting across the interface.
The aforementioned object is achieved by a special adhesion promoting coating of the metal, i.e., a metal reinforcement element for polymeric or elastomeric materials with a coating of a polymer or prepolymer deposited from an aqueous, alcoholic or organic solution and compatible with and co-polymerizable, co-vulcanizable or crosslinkable with said polymeric or elastomeric material to be reinforced, and additionally bearing functional groups, which are either covalently bonding to the metal surface of said reinforcement element or which form covalent bonds with the outward directed first functional groups of a mono- or multimolecular layer of a bifunctional adhesion promotor intercalated between said metal and said coating and bound to said metal by its second functional groups.
There is provided a method for coating a metal surface, more particularly a metal reinforcement element, comprising the steps of depositing an adhesion promoter, from an aqueous, alcoholic, or organic solvent; depositing a functionalized non-cured polymer or prepolymer from an aqueous, alcoholic, or organic solvent, or from the bulk material; which is carried out in a one step procedure or subsequently.
The adhesion promotor is advantageously utilized in a method for coating a metal surface with an adhesion promotor, comprising the preparation of a solution of the adhesion promotor and the dipping or painting of said metal surface with said solution, wherein the adhesion promotor is an organosiloxane of the general formula (I) given below, wherein X is a siloxane group, a chlorosilane group or a bromosilane group, and especially: xe2x80x94SH; xe2x80x94SiHCl2; xe2x80x94SiH2Cl; xe2x80x94Si(Cl)3; xe2x80x94SiHBr2; xe2x80x94SiH2Br; xe2x80x94SiBr3; xe2x80x94Si(Rxe2x80x2(Cl)2); xe2x80x94Si(ORxe2x80x2)3; xe2x80x94Si(Rxe2x80x2(ORxe2x80x2)2); COOH; COCl.
The adhesion promotor is pretreated before use in the following manner: a definite amount of water is added under stirring at elevated temperature, the amount being calculated to at least partially hydrolyse and to partially condense the siloxane. The solution is diluted with alcohol and immediately after a certain ripening period applied to said metal surface, thereby forming a dense mono- or multimolecular layer on the metal surface.
The preparation of the organosiloxane is a step which was found to yield excellent dense coatings of the organosilane adhesion promotor that is intercalated between the metal surface and the polymer matrix of the coating. This results in an excellent performance regarding aging and corrosion of the metal surface.
The pretreatment preferably includes the hydrolyzation of a catalytic amount of the organosiloxane and dilution of this starter solution with diluent, water and the siloxane. Thereby, a partial hydrolyzation of the organosiloxane is achieved.
This solution is diluted with alcohol and immediately after a certain ripening period, applied to the metal surface. Preferably the organosiloxane is 3-amino-propyl-trimethoxysilane.
The metal surface can be treated to clean and/or at least partially oxidize the surface. Preferably the metal surface Is treated with isopropanol (iPrOH), a mixture of isopropanol with water (iPrOHxe2x80x94H2O), or diluted aqueous HCl. It is also possible to treat the surface with KOH or with a carbonate. All these measures can be used alone or in combination with each other.
The method for coating a metal surface, especially of a coated metal reinforcement element, comprises the deposition of an adhesion promotor, especially by use of the pretreated organosiloxane solution, from an aqueous, alcoholic, or organic solvent, or from the bulk material, and the deposition of a functionalized non-cured polymer or prepolymer from an aqueous, alcoholic, or organic solvent in a one step procedure or subsequently.